If two or more things are interdependent, each one needs the others, and the others need it. They are tied together, not just sitting side by side. Countries can be interdependent when they trade; family members can be interdependent in daily life; the heart, lungs, and blood are interdependent because each keeps the others working. This is the adjective that matches the noun interdependence. It is a careful, rather school-like word. Though actually, people do not usually use it for a small, one-way favour (“she helped me once”). They use it when the need really runs both ways, and when breaking the link would hurt more than one side.